In a direct injection internal combustion engine, fuel is pressurized in a high-pressure fuel pump and delivered from the high-pressure fuel pump to a delivery pipe, which retains the fuel, and then distributed from the delivery pipe to injectors of respective cylinders, thus supplying the fuel to the engine. The fuel delivery capacity of the high-pressure fuel pump may be lowered by increase in fuel leakage as the pump deteriorates over time. The lowered fuel delivery capacity of the high-pressure fuel pump causes insufficient fuel supply when the engine is in a full load operating state, which maximizes the amount of requested fuel supply. As a result, the air-fuel ratio of air-fuel mixture burned by the engine becomes lean, thus causing misfire, engine stall, or excessively heated catalyst.
Conventionally, to solve this problem, Patent Document 1 proposes a technique to avoid insufficient fuel supply by limiting the opening degree of the throttle valve of an internal combustion engine, which is the throttle opening degree, when it is determined that the fuel delivery capacity of a high-pressure fuel pump has lowered through deterioration of the pump over time. In an internal combustion engine performing air-fuel ratio control, the fuel supply amount is adjusted in correspondence with the intake air amount such that the air-fuel ratio of the air-fuel mixture delivered into a combustion chamber matches a target value. Accordingly, by decreasing the intake air amount through limitation of the throttle opening degree, the fuel supply amount necessary for ensuring an optimal air-fuel ratio is reduced. As a result, a lean air-fuel ratio is avoided even after the fuel delivery capacity of the high-pressure fuel pump is lowered.